AST 301Introduction to AstronomyJohn LacyRLM [email protected] LiRLM [email protected] JeonRLM [email protected] site: www.as.utexas.eduGo to Department of Astronomy courses,AST 301 (Lacy), course websiteTopics for this weekHow do the life stages of a high-mass star differ, and why?Describe the events that lead up to the two types ofsupernova.What is a white dwarf?What is a typical white dwarf mass, radius, composition?How does pressure in a white dwarf differ from normalgas pressure?What is a neutron star?How big is it, what is it made of?How could one form?What is a black hole?Office hoursI will be at a meeting during my normal office hours thisweek.I will answer questions after class.I will have office hours this afternoon 2-3:00.I will not have office hours tomorrow.How were the atoms in your body made?The hydrogen atoms (or the protons and electrons they aremade of) were made in the big bang.Many of the helium atoms in the Universe were also madein the big bang.The other atoms were made inside of stars or duringexplosions of stars.When the Sun becomes a red giant, carbon and maybeoxygen will be made in its core.But the core will be the left-over white dwarf.The gas put back out into space will come from the redgiant’s envelope, which hasn’t been hot enough forfusion to make new elements.Most of the elements in space were put there by supernovaexplosions.After the planetary nebulaA planetary nebula can be seen for about 10,000 years.After that, it expands and becomes faint.And the central star (the core of the red giant) cools off andno longer lights it up.The central star is then called a white dwarf.Why does the white dwarf cool off?It has lost its envelope, which served as a blanket to keepits heat in.But that means it can lose heat faster. Shouldn’t it thencontract and heat up?It turns out it doesn’t contract because it is no longer anormal gas.Normal gas and degenerate gasPressure is the force a gas exerts on its surroundings.It is caused by the motion of the atoms.Or in a star, it is mostly the free electrons that causepressure, since it is too hot for atoms to hold onto theirelectrons.In a normal gas, the electrons’ motion is caused by heat.But at very high densities, the wave properties of theelectrons become important, and the electrons mustmove fast even if the temperature is low.The result is that when a white dwarf loses energy, itselectrons don’t slow down, so its pressure doesn’tdecrease, so it doesn’t contract.So it cools off instead of heating up.White DwarfsBecause the pressure of degenerate electrons doesn’tdecrease when they lose energy, the core of the redgiant doesn’t contract, and so it doesn’t heat up.It simply cools off.Fusion stops and never starts up again.It is then a white dwarf.White dwarfs have masses ½ - 1 times that of the Sun.They start out very hot, about 100,000 K, but cool off.Their sizes are about like that of the Earth.Density of matter in a white dwarfDensity = mass / volumeThe density of the Sun is about equal to the density of water,and the mass of a white dwarf is about equal to the massof the Sun.The radius of a white dwarf is about 100 times smaller thanthe radius of the Sun.How does the volume of a white dwarf compare to thevolume of the Sun?A. 100 times smallerB. 1,000 times smallerC. 10,000 times smallerD. 1,000,000 times smallerDensity of matter in a white dwarfDensity = mass / volumeThe density of the Sun is about equal to the density of water,and the mass of a white dwarf is about equal to the massof the Sun.The volume of a white dwarf is about 1,000,000 timessmaller than the volume of the Sun.How does the density of a white dwarf compare to thedensity of the Sun?Density of matter in a white dwarfDensity = mass / volumeThe density of a white dwarf is about 1,000,000 times thedensity of the Sun.The density of a white dwarf is about 106 grams/cubic cm,or 1 ton/cm3, or 16 tons/cubic inch.The discovery of pulsarsJocelyn Bell, a student in England, was observing ‘radiostars’ with a radio telescope in 1967.She noticed that one of the stars seemed to flickerregularly.Perhaps jokingly, they at first thought it was a signal froman extraterrestrial civilization, but soon other stars like itwere found, and they concluded that it was a naturalphenomenon.(The professor she was working for got the Nobel Prize forthe discovery.)How can a star flash 30 times a second?Even if the Sun could turn on and off in 1/30 second, itsradius is about 2 light-seconds, so it wouldn’t appear tous to all turn on and off together.White dwarfs are small enough to avoid this problem, butwhat could make them flash?We know of pulsating stars that vary in brightness byvarying in size, but they take minutes to years to vary.They also don’t turn off between flashes like pulsars do.We think instead that pulsars work like lighthouses, with alight that doesn’t turn on and off, but rotates around.Lighthouse model of apulsarCan a star rotate 30 times per second?A star can’t rotate faster than the time for a satellite wouldtake to orbit near its surface.Otherwise the gas near the surface of the star would gointo orbit.So the Earth can’t rotate in less than 90 minutes.The Sun can’t rotate in less than 3 hours.A white dwarf can’t rotate in less than about 10 seconds.But neutron stars are so compact that they can rotate 1000times a second without flying apart.They also have strong magnetic fields to direct theirbeacons.Density of matter in a neutron starDensity = mass / volumeThe radius of a neutron star is about 105 times smaller thanthe radius of the Sun.How does the volume of a neutron star compare to thevolume of the Sun?A. 105 times smallerB. 3x105 times smallerC. 108 times smallerD. 1015 times smallerDensity of matter in a neutron starDensity = mass / volumeThe density of the Sun is about equal to the density of water,and the mass of a neutron star is somewhat more than themass of the Sun.The volume of a neutron star is about 1015 times smaller thanthe volume of the Sun.How does the density of a neutron star compare to thedensity of the Sun?Density of matter in a neutron starThe density of a neutron star is about 1015 times the densityof the Sun (or about 1015 times the density of water).The density of a neutron star is about 1015 grams / cubic cm,or 109 ton / cm3, or 1 ton / cubic hair.Why do neutron stars rotate so fast?What happens
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